1. Field of the Invention
The present invention relates to a method and an apparatus for controlling the air-fuel ratio in an internal combustion engine.
2. Description of the Prior Art
In general, the air-fuel ratio in an internal combustion engine of a motor car is selected to be equal to or less than the stoichiometrical air-fuel ratio in the ordinary running state; to be equal to the value, approximately 13, corresponding to the maximum output of the engine in the accelerating state with a wide open throttle and in the slope ascending running state; and to be equal to the value chosen from the viewpoint of the stability of the engine in the idling state.
In the prior art, for air-fuel ratio control in the ordinary running state, an open loop control of the carburetor is used in which some loss occurs in the specific fuel consumption due to production line variations in the structure of the engine, the long term variation in the operational characteristic of an engine, and to production line variations in the structure of the carburetor. In an electronically controlled fuel injection device, in which the volume of the intake air is measured by an intake air volume sensor and the like, the required amount of fuel is calculated by a computer device, and the fuel is injected into the air intake manifold by a solenoid valve in accordance with the calculated required amount, a closed loop control is used in which the direction of the stoichiometrical air-fuel ratio (approximately 15) is determined by an oxygen concentration sensor in the exhaust duct. Also, closed loop control of the carburetor, in which the amount of air to be bled is modified in accordance with the determination of the direction of the stoichiometrical air-fuel ratio by the oxygen concentration sensor, is used for some engines. Although these closed loop control systems can reduce variations in the air-fuel ratio, they are disadvantageous in that fuel consumption becomes higher because the theoretical air-fuel ratio is not the correct air-fuel ratio for the best specific fuel consumption.
It is known that, in order to prevent the above described fuel consumption loss from occurring and to attain the optimum specific fuel consumption, a constant altering (so-called "dithering") of the flow rate of the air which by-passes the carburetor is carried out, i.e., the air-fuel ratio is varied at predetermined periods between the rich and lean sides alternately. In this way the direction of the air-fuel ratio which gives the best improvement of the specific fuel consumption is determined, and the air-fuel ratio is then corrected by the subsidiary air valve which by-passes the carburetor. In this method, the engine running is effected once in an air-fuel ratio at the relatively richer side level and once in another air-fuel ratio at the relatively leaner side level, and the rotation rate N.sub.er obtained by running under the richer side air-fuel ratio and the rotation rate N.sub.el obtained by running under the leaner side air-fuel ratio are compared. The control of the engine is carried out in such a manner that, if N.sub.er &gt;N.sub.el, the amount of by-pass air is decreased, while if N.sub.er &lt;N.sub.el, the amount of by-pass air is increased. (For example, as disclosed in Japanese Unexamined Patent Publication (Kokai) No. 57-46045.)
In the prior art, the number of operation points for detecting the signals for the running states, such as the engine rotational speed, engine torque, or related states, was selected as at least three, and the correction of the air-fuel ratio carried out by changing the required amount of fuel.
In the prior art, in order to attain a precise separation between the engine rotational speed change due to the accelerator pedal actuation by the driver, changes in road conditions, and the like, and that due to the constant altering of the air-fuel ratio for optimum control, that is the change of the air-fuel ratio alternately to the rich side and to the lean side at a predetermined period, it is desirable to increase the number of operation points for detecting the signals for the running states. However, if the number of operation points is increased, the opportunities for correcting the air-fuel ratio are correspondingly reduced, leading to an unavoidable deterioration in the specific fuel consumption.